1. Field of the Invention
The present invention relates, generally, to a separator plate for a molten carbonate fuel cell and a manufacturing method thereof. More particularly, the present invention relates to a separator plate for use in a molten carbonate fuel cell, which functions to reform a fuel gas while allowing it to efficiently flow therein and thereout, thus producing hydrogen and carbon dioxide, which are then supplied into an anode, and which functions to realize the electrical connection between the anode and the cathode, and to a method of simply manufacturing the same.
2. Description of the Related Art
Fuel cells are receiving attention as a next-generation generator having high efficiency and generating little pollution for converting chemical energy into electrical energy through the oxidation-reduction reaction of reactants.H2+CO32−H2O+CO2+2e− (Anodic Oxidation)½O2+CO2+2e−CO32− (Cathodic Reduction)
The fuel cell is essentially composed of an anode, a cathode, and an electrolyte matrix positioned between the anode and the cathode, in which an electrolyte is incorporated in the electrolyte matrix to assure efficient ion flow. That is, a fuel gas is supplied into the anode to thus oxidize it, whereas oxygen or air is supplied into the cathode along with carbon dioxide to generate carbonate ions (CO32−), which are then transferred to the anode from the cathode through the electrolyte matrix positioned between the anode and the cathode. As such, electrons flow via an external circuit. Thus, in the fuel cell, the chemical energy is directly converted into electrical energy through the oxidation-reduction reaction of hydrogen and oxygen. Accordingly, the fuel cell is advantageous in that it has high efficiency (since there is no limitation like that of a Carnot cycle, which is characterized by low generation efficiency when mechanically generating heat by heating water or other media and rotating a turbine using steam pressure, as in typical heat generation), generates little pollution (since nitrogen oxide or sulfur oxide are not discharged), produces no noise (since there are no driving parts), can be made modular (since the fuel cell is easy to construct and enlarge and the capacity thereof may be variously formed), is compatible with a variety of fuels (since it is possible to use fuels such as hydrogen, coal gas, natural gas, methanol, and gasoline), and enables cogeneration (since warm water may be produced using waste heat in a high-temperature fuel cell).
In particular, called a second generation fuel cell, a molten carbonate fuel cell (hereinafter, referred to as an “MCFC”) is characterized in that material, in which carbonate of alkali metal such as lithium carbonate or potassium carbonate is melted, is used as an electrolyte, and sintered nickel and sintered lithiated nickel oxide are used as an anode and a cathode, respectively. That is, a fast electrochemical reaction at high temperatures enables the use of inexpensive nickel, instead of platinum, as electrode material, thus generating economic benefits. Further, thanks to the properties of the nickel electrode in which even carbon monoxide, which negatively affects the platinum electrode, may be used as fuel through a water gas shift reaction, various fuels, such as coal gas, natural gas, methanol, and biomass, may be selected. When good quality high-temperature waste heat is recovered using a heat recovery steam generator (HRSG), the total heat efficiency of the generation system may be increased to about 60% or higher. Furthermore, since the MCFC is operated at high temperatures, an electrochemical reaction and a fuel reforming reaction may simultaneously take place in a fuel cell stack to thus realize internal reforming. Such an internal reforming MCFC functions to directly apply the heat value of the electrochemical reaction to a reforming reaction, which is an endothermic reaction, even without the use of an additional external heat exchanger, and therefore the total heat efficiency of the system is much higher than that of an external reforming MCFC, and furthermore, the structure of the system is simplified. The MCFC is composed largely of a stack for producing electricity, a mechanical peripheral device, such as a fuel supplier, and an electrical peripheral device, such as an electrical converter. In particular, since the stack affects the generation efficiency, lifetime, and performance of the MCFC, thorough research into shapes of separator plates constituting the stack and methods of supplying fuel into the separator plate has been conducted.
Despite such advantages of the MCFC, it is disadvantageous because it must be operated at high temperatures and uses highly corrosive molten carbonate as an electrolyte, undesirably leading to easy corrosion of the constituents of the cell. In particular, the separator plate should be provided with a cathode part, an anode part, and an electrolyte matrix therebetween, and a fuel gas and an oxidizing gas should separately flow in the separator plate, and thus the corrosion of the separator plate or the leakage from the separator plate very negatively affect the overall performance of the fuel cell. In addition, the separator plate of the MCFC should function to reform a fuel gas, such as natural gas or coal gas, which is continuously supplied, into hydrogen.
A conventional separator plate for an MCFC stack (Korean Examined Patent Publication No. 10-0259213) comprises an anode manifold for transferring hydrogen and carbon dioxide, reformed from a fuel gas using a reformer, as a fuel converter among peripheral devices, to the separator plate, and a cathode manifold for transferring air and carbon dioxide, the anode manifold and the cathode manifold being sequentially disposed on the same surface of the separator plate. In addition, in order to maintain gas tightness, the separator plate has a structure in which the electrolyte matrix between the ends of the manifolds has a wet seal area for preventing the supplied gases from mixing, and the gases are transferred to the anode and the cathode via a shielded slot type current collector plate, ultimately causing an electrochemical reaction. The current is produced by the oxidation-reduction generated from the anode and the cathode, and thus flows along the separator plate. In the separator plate having such a structure, it is very important that the mixing of gases respectively distributed to the anode and the cathode be prevented in order to realize gas tightness. Hence, a welding process is performed a greater number of times, and thereby the form of the separator plate is complicated, undesirably increasing the manufacturing cost. Even though heat treatment is conducted, since the distortion of the separator plate is not completely solved, upon stack lamination, uniform stress distribution and an airtight wet seal area cannot be realized. Moreover, using the reformer, which is an external peripheral device, the fuel gas is reformed to supply hydrogen, and therefore it is impossible to control the temperature distribution that forms in the gas supplied into the stack.
In a conventional separator plate, with the goal of completely separating the gas of the anode part from that of the cathode part, the end of the separator plate and the gas inlet of the manifold are welded using an Nd-YAG laser, and the wet seal area is subjected to corrosion resistant coating using a mixture comprising aluminum, as a main ingredient, nickel, titanium, chromium, and copper, or using a ceramic material such as titanium nitride, and is then allowed to stand at 500˜600° C. for a predetermined time period in a reduction atmosphere or in a vacuum furnace, followed by performing heat treatment for forming an aluminum diffusion layer at an increased temperature of 700˜850° C.
Since a hot-dip process, which is a conventional coating process, is difficult to use to mask an undesired portion and must be performed at high temperatures, the deformation of base metal undesirably occurs after using an aluminum melt. In addition, although a physical vapor deposition process enables the formation of a high quality coating layer, it suffers because the thickness of the layer is difficult to increase and the preparation cost thereof is very high. In addition, a pack cementation process may cause problems related to the deformation of a separator plate and the phase change of a base metal when work is conducted at 1000° C. or higher. Further, in the case of a thermal spray process, due to blasting for pretreatment or the pressure of a gun, a base metal may be deformed or pores may remain therein, and the thickness of the layer may be non-uniform. Furthermore, a slurry coating process is inexpensive and is easy to use to coat various shapes, but it is difficult to maintain the viscosity of the slurry, and thus the thickness uniformity is decreased, and also pores, resulting from solvent evaporation, are difficult to eliminate, and thereby the thickness of the coating layer is limited.
Accordingly, the present invention proposes a separator plate which is different from a conventional separator plate for an MCFC, and specifically a direct internal reforming (DIR) MCFC, which can be manufactured through a simple process and has a long stack lifetime, and to which a screen printing coating process is applied.